This invention relates to solenoidal electric field lamps and, in particular, to such lamps containing one or more patches of amalgam material for the control of mercury vapor pressure.
In general, the efficacy of fluorescent lamps is a function of the mercury vapor pressure within the lamp envelope. This pressure in turn is usually a function of the coldest part of the lamp envelope. However, more recently developed solenoidal electric field (that is, SEF) lamps are more compact than conventional, tubular fluorescent lamps having the same power rating. In these SEF lamps, the coldest temperature spot on the envelope wall is generally higher than desirable for optimum lamp efficacy. This is due to the fact that the higher minimum wall temperature increases the mercury vapor pressure.
One solution to this problem is to place a material which amalgamates with mercury in the lamp envelope. For a further discussion on amalgams and amalgam positioning within SEF lamps, see U.S. Pat. No. 4,262,231, issued Apr. 14, 1981 to John M. Anderson and Peter D. Johnson. This patent provides a detailed discussion of temperature distribution within SEF lamps and, accordingly, it is hereby incorporated herein by reference, as background material.
One of the materials which is employed as a mercury amalgam is indium. At any given temperature, mercury vapor pressure over the mercury-indium amalgam is lower than mercury vapor pressure over a pool of pure mercury at the same temperature. The temperature of the amalgam "pool" may therefore be used to control mercury vapor pressure.
The presence of the amalgam material reduces the mercury vapor pressure in a hot lamp but, however, it also unfortunately reduces the mercury vapor pressure when the lamp is cool, that is, at room temperature (20.degree. C., for example). This creates two problems. First, the lamp starting voltage is increased and second, the lamp efficacy is decreases until the amalgam is brought up to lamp operating temperature.
In some conventional fluorescent lamps, this problem is solved by placing some of the amalgam-forming material at a location within the lamp where it can be heated quickly by some part of the lamp. For example, in conventional fluorescent lamps, some amalgam-forming material may be placed on a ring formed around at least one of the electrodes. The heat dissipated by the electrode heats the ring and quickly brings the mercury amalgam to the desired temperature. The heated ring quickly supplies mercury vapor for the lamp discharge. Under steady state conditions, the amalgam on the heated ring is typically too hot for optimum lamp efficacy and mercury pressure control is automatically transferred to a second mercury amalgam location. Such a system is described on pages 59-60 of "Fluorescent Lamps" by W. Elenbass (McMillan Press Ltd., London, 1971). While such a method is satisfactory for mercury vapor pressure control in lamps having heated electrodes, it is not applicable to the electrodeless discharge configuration found in SEF lamps. In such lamps there are no heated electrodes.
The utility of amalgam heating in fluorescent lamp structures also appears to have been appreciated in U.S. Pat. No. 3,336,502, issued to Leland W. Gilliatt. However, this patent only appears to disclose an external means for heating amalgams in particular types of fluorescent lamps. There is no appreciation of the particular problems presented by the compact SEF lamp structure and design. Furthermore, in U.S. Pat. No. 3,898,511, there is also apparently disclosed a mercury amalgam heating system in which the amalgam is activated by heat radiated from lamp electrodes.